Introduce category for compositions that should have default length on insertion.

Related to #549

data/transitions/dissolve.xml

 <!DOCTYPE kpartgui>
-<transition tag="luma" id="dissolve">
+<transition tag="luma" id="dissolve" type="short">
     <name>Dissolve</name>
     <description>Fade out one video while fading in the other video.</description>
     <parameter type="list" name="resource" default="" paramlist="%lumaPaths" optional="1">

data/transitions/slide.xml

 <!DOCTYPE kpartgui>
-<transition tag="composite" id="slide">
+<transition tag="composite" id="slide" type="short">
         <name>Slide</name>
         <description>Slide image from one side to another.</description>
         <author>Dan Dennedy</author>

data/transitions/wipe.xml

 <!DOCTYPE kpartgui>
-<transition tag="composite" id="wipe">
+<transition tag="composite" id="wipe" type="short">
         <name>Wipe</name>
         <description>Applies a stationary transition between the current and next frames.</description>
         <author>Dan Dennedy</author>

src/timeline2/view/timelinecontroller.cpp

@@ -405,8 +405,10 @@ int TimelineController::insertNewComposition(int tid, int clipId, int offset, co
             }
         }
     }
-    if (duration < 0) {
-        duration = pCore->currentDoc()->getFramePos(KdenliveSettings::transition_duration());
+    int defaultLength = pCore->currentDoc()->getFramePos(KdenliveSettings::transition_duration());
+    bool isShortComposition = TransitionsRepository::get()->getType(transitionId) == TransitionType::VideoShortComposition;
+    if (duration < 0 || (isShortComposition && duration > 1.5 * defaultLength)) {
+        duration = defaultLength;
     } else if (duration <= 1) {
         // if suggested composition duration is lower than 4 frames, use default
         duration = pCore->currentDoc()->getFramePos(KdenliveSettings::transition_duration());

src/transitions/transitionsrepository.cpp

@@ -97,6 +97,8 @@ void TransitionsRepository::parseCustomAssetFile(const QString &file_name, std::
         QString type = result.xml.attribute(QStringLiteral("type"), QString());
         if (type == QLatin1String("hidden")) {
             result.type = TransitionType::Hidden;
+        } else if (type == QLatin1String("short")) {
+            result.type = TransitionType::VideoShortComposition;
         }
         customAssets[result.id] = result;
     }
@@ -164,7 +166,7 @@ std::unique_ptr<Mlt::Transition> TransitionsRepository::getTransition(const QStr
 bool TransitionsRepository::isComposition(const QString &transitionId) const
 {
     auto type = getType(transitionId);
-    return type == TransitionType::AudioComposition || type == TransitionType::VideoComposition;
+    return type == TransitionType::AudioComposition || type == TransitionType::VideoComposition || type == TransitionType::VideoShortComposition;
 }
 
 const QString TransitionsRepository::getCompositingTransition()

src/transitions/transitionsrepository.hpp

@@ -33,7 +33,7 @@
  * Note that this class is a Singleton
  */
 
-enum class TransitionType { AudioComposition, VideoComposition, AudioTransition, VideoTransition, Favorites, Hidden = -1 };
+enum class TransitionType { AudioComposition, VideoShortComposition, VideoComposition, AudioTransition, VideoTransition, Favorites, Hidden = -1 };
 Q_DECLARE_METATYPE(TransitionType)
 
 class TransitionsRepository : public AbstractAssetsRepository<TransitionType>